Adjustable microwave phase shifter using stripling having variable dielectric



June 30, 1964 FRENCH ETAL 3,139,597

R. V. ADJUSTABLE MICROWAVE PHASE SHIFTER USING STRIPLINE HAVING VARIABLE DIELECTRIC Filed Dec. 8, 1960 3 Sheets-Sheet 1 Fig. I

INVENTOR$ RICHARD V. FRENCH ROBERT A.JORDAN ERNEST J. WILKINSON ATTORNEY June 1964 R. v. FRENCH ETAL 3,139,597

ADJUSTABLE MICROWAVE PHASE SHIFTER USING STRIPLINE HAVING VARIABLE DIELECTRIC 3 Sheets-Sheet 2 Filed Dec. 8, 1960 E O O [lg f -46 la 40 Fig.3

INVENTORS RICHARD V. FRENCH ROBERT A. JORDAN ERNEST J. WILKINSON ATTORNEY June 30, 1 64 R. v. FRENCH ETAL 3,139,597

ADJUSTABLE MICROWAVE PHASE SHIFTER USING STRIPLINE HAVING VARIABLE DIELECTRIC Filed Dec, 8, 1960 3 Sheets-Sheet 5 INVENTORS RICHARD v. FRENCH By ROBERT A. JORDAN RNEST J. wuu mso- ATTORNEY specification.

United States Patent 3,139,597 ADJUSTABLE MECROWAVE PHASE SIHFTER USEN G STRHLINE HAVING VARIABLE Di- ELEQTRIC Richard V. French, Saiem, N.H., and Robert A. Jordan,

Essex, and Ernest J. Wiikinson, Westwood, Mass, as-

signors to Sylvania Electric Products Inc., a corporation of Deiaware Filed Dec. 8, 1960, Ser. No. 74,536 5 Claims. (til. 333-3l) ample, of large phased array radar systems, where, for

purposes of regulating the phase of the transmitted signal, the electrical length of the transmission lines from the transmitting equipment to the several radiating elements in the antenna array must be equal. Disregarding frequency, the electrical length of a transmissionline generally is dependent upon its physical length, and, accordingly during fabrication, the line is laid out and measured to a length that corresponds to the desired electrical Because of inherent factors such as line sag, stretch, and human errors in measurement, this procedure is only good enough to give a course approximation of the specified electrical length. Other techniques must be employed to bring the system into alignment once the lines have been installed. Compensating devices, such as phase shifters, are therefore used in the low-power sections of the system to change the apparent electrical length of the lines requiring alignment.

Although a wide variety of prior art phase shifting devices are available for changing the apparent electrical length of transmission line, many of them have some undesirable characteristics which limit their usefulness. For example, in one type of prior art phase shifter, commonly known as the trombone or line-stretcher type, phase shift is produced by mechanically elongating or shortening the effective length of the signal conductor, by telescoping two sections of transmission line. Since the device requires the use of sliding contacts to mate the variable junction of the lines, the device, due to'wear caused by the sliding action, will in time develop undesirable loss and impedance characteristics that make the device unsuitable for continuous use in the system. Another shortcoming of the trombone type device, or any other mechanical phase shifter that employs linear displacement to vary signal phase, is its undesirably long length, particularly in applications requiring appreciable phase shift. Other mechanical phase shifters of which applicants are aware require some motion of the electrical conductor, or the use of a large number of moving parts, or involve complex mechanical arrangements, making them expensive and susceptible to the development of impedance discontinuities and insertion losses, and unfit for prolonged use.

Another phase shifting device known to applicants, a so-called variable dielectric phase shifter, operates on the principle that the phase velocity of an electromagnetic Wave propagated in a dielectric medium varies inversely as the square root of the dielectric constant of that medium. Briefly stated, the device includes a fixed length section of strip transmission line centrally positioned between two dielectric materials having different dielectric constants, each being supported on a respectivesemi-circular grounding plate. The two sets of grounding plates lie in planes parallel to the plane of the strip conductor 3,139,597 Patented June 30, 1964 and are attached to a rotatable shaft passing perpendicularly through the device. Because the dielectric media in the device is not uniform, different spacing between the grounding planes is utilized to produce a uniform characteristic impedance through the device. The shaft is supported in a hearing which permits rotation of the two sets of grounding plates and their corresponding dielectrics as a unit about the axis of the shaft. The strip conductor is shaped to lie within a semi-circular area so that its surface may be entirely covered by either of the two semi-circular dielectric materials, or a variable percentage of both, as the shaft is rotated. Since the amount of phase shift produced by the device is proportional to the length of strip conductor contained therein,the strip conductor may have a labyrinth contour in order to locate a conductor of maximum length within the limited semi-circular layout area.

The phase differential between the input and output terminals, being dependent upon the signal propagation velocity of the device, is determined by the relative positions of the dielectric medium and the strip conductor. When the angular position of the grounding plates is such that the dielectric material having the lower dielectric constant alone covers the area occupied by the strip conductor, the propagation velocity is maximum, with the result that a signal experiences minimum phase shift between the two terminals. As the grounding plates are rotated from this position of minimum phase shift, more and more of the strip conductor is exposed to the higher constant dielectric material, thereby decreasing the propagation velocity and increasing the phase shift. When the plates are rotated to the position where the entire section of strip conductor is contained within the higher dielectric constant material, the propagation velocity of the device is minimum, with attendant maximum phase shift provided between the inut and output terminals.

While this prior art device, now disclosed in US. Patent No. 3,005,168, issued October 17, 1961, provides an acceptable phase shift for many applications, certain structural features seriously limit its practicality and usefulness. Since the dielectric material on both sides of the strip conductor must be moved with respect to the strip conductor to achieve an adjustment of the phase shift, the conductor cannot be supported on the dielectric (as is usual in strip transmission line construction), but must be otherwise supported. One approach has been to form the inner conductor of a fiat rod-like member, supported at points outside the area of the rotatable grounding plates so as not to interfere with their motion. This construction is awkward, however, susceptible to vibration, and otherwise undesirable.

With an appreciation of the foregoing limitations of phase shifting devices in general, and the shortcomings of the prior art variable dielectric type phase shifter in particular, applicants have as the primary object of this invention to provide an improved microwave phase shifting device.

Another object of the invention is to provide an improved adjustable microwave phase shifter having a constant characteristic impedance over the range of operation of the device.

Another object of the present invention is to provide a microwave phase shifter capable of producing variable and continuous phase shift in response to a simple meprovide a reliable microwave phase shifter having the foregoing features and advantages, yet of simple and inexpensive construction.

Another object of the invention is to provide an improved variable dielectric type phase shifter which is resistant to shock and vibration.

These objects are attained, in accordance with the invention, by embodying the basic principles of prior art dielectric type phase shifters in an improved structure featuring mechanical simplicity, compactness and rigidity. Briefly, the device consists of a section of strip transmission line including an inner conductor positioned between upper and lower transmission planes. The lower transmission plane comprises a ground plate and a block of dielectric material on which the strip inner conductor is supported. The upper transmission plane includes a circular ground plate supported for rotation directly above the inner conductor, and spaced therefrom by two or more dielectrics having differing dielectric constants, the dielectrics being rotatable with the plate. When the upper plate is positioned such that the dielectric having the lower dielectric constant covers the entire area occupied by the conductor, the propagation velocity through the device is maximum and a minimum phase shift results. As the upper grounding plate is rotated from this setting, a greater portion of the area containing the inner conductor is exposed to the dielectric having the higher dielectric constant, resulting in an increase in phase shift. Finally, when the entire area of the inner conductor is covered only by the higher constant dielectric, the propagation velocity reaches a minimum and maximum phase shift is achieved. Maximization of the influence that the dielectrics in the upper transmission plane have on phase shift, and attendant extension of the operating range of the device, is achieved by locating the inner conductor closer to the upper ground plate. The strip conductor is bonded to the stationary lower dielectric, making the device much more resistant to shock and vibration than prior art phase shifters of the dielectric type. Phase adjustment involves motion of the upper grounding plate only, thus eliminating the need for the periodic alignment of the grounding plates required with the prior art dielectric phase shifter alluded to above.

Other objects, features, and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an exploded isometric view of a preferred embodiment of the invention;

FIG. 2 is an isometric view of the under side of the circular portion of the upper grounding plate, portions thereof being exaggerated for clarity;

FIG. 3 is a top plan view of the device, with the circular upper grounding plate removed to illustrate the location of the inner conductor;

FIG. 4 is an elevation cross-sectional side view of the device taken along line 4-4 of FIG. 3, including the circular upper grounding plate; and

FIGS. 5A and 5B are schematic diagrams useful in explaining the benefits of locating the strip conductor nearest the upper grounding plate.

Referring to FIG. 1, the present phase shifter consists essentially of a section of strip transmission line of controllable electrical length including a ground plate 10 on which is supported a block or slab of dielectric material 12, for example, modified polystyrene, sold by Dow Chemical Co. under the trade name Styron 475, of thickness t Printed or otherwise bonded to the top surface of dielectric slab 12 is a flat conductor 14 of strip or foil-like configuration which constitutes the signal conductor of the transmission line. Since, as will be shown later, the phase shifting range of the device is a function of the physical length of conductor 14, its length between its terminating sections 14a and 14b is elongated by employing the illustrated recurvate contour layout. The

contour is not restricted to that shown, however, but may be of arcuate or triangular outline, or follow a sinuous course back and forth about an arcuate path. For reasons which will become apparent, the length of conductor 14 is confined to a semi-circular area, represented by the dotted line 16, the diameter of which lies on the axis through the terminating sections 14a and 14b.

The upper ground plate 18 of the strip transmission line is spaced from conductor 14 by a second slab of dielectric material 20. The slab of dielectric material 20 may have the same dielectric constant as slab 12, and for reasons to be explained later, preferably has a thickness less than the thickness of slab 12, designated i Conductive plate 18 and dielectric slab 20 have a circular opening 22 therein having a diameter corresponding to the diameter of the semi-circular area 16 occupied by conductor 14. As best seen in the cross-sectional view of FIG. 4, and in FIG. 3, the opening in plate 18 is surrounded by a narrow shoulder 24, the top surface of which is carefully machined and lies in a plane parallel to the surfaces of plate 18.

Adjustment of the phase shift of the device is provided by the assembly 26 which includes a circular conductive plate 28, the outer diameter of which corresponds to the outer diameter of the shoulder 24 surrounding the opening 22 in plate 18. The plate 28 is formed with a pcripheral lip 28a having an inner diameter corresponding to the diameter of opening 22, and a thickness substantially equal to spacing of the shoulder 24 from the upper surface of plate 18. The shoulder 34 and under surface of the lip are carefully machined to permit rotation of plate 28 relative to plate 18. Such rotation is facilitated by a shaft 30 secured centrally of plate 28 and provided with a control knob or gear 32.

The under side of plate 28 is divided along a diameter into two sectors 28b and 280 having fiat surfaces offset from each other but both parallel to the surfaces of plate 18. As best seen in FIGS. 2 and 4, a fiat semi-circular plate of dielectric material 34 is secured by screws 36 over the sector 280, the surface of which is further from conductor 24 than the surface of sector 28b. The thickness of the dielectric slab 34 is greater than the amount of offset between surfaces 28b and 280 with the result that the lower surface of the dielectric slab 34 lies in a plane below the surface of sector 28b. Further, the thickness of dielectric slab 34 is so related to the dimension between the shoulder 24 and the upper surface of dielectric slab 12 that when plate 28 is placed in the opening the lower surface of dielectric slab 34 just clears conductor 14. Thus, plate 28 may be rotated with respect to plate 18 without interfering with the inner conductor 14. When assembled, the assembly 26 provides in the vicinity of conductor 14 two different dielectrics, the semi-circular dielectric slab 34 having a dielectric constant determined by the material of which it is formed, and an air gap of semi-circular shape having a dielectric constant of one.

In stripline transmission lines, of which the abovedescribed structure is a special example, the impedance is a function of the width and thickness of the center conductor, the spacing between the ground planes, and the effective dielectric constant between the ground planes. The velocity of propagation along the line is a function only of the effective dielectric constant between the ground planes. In order to maintain a constant impedance while changing the effective dielectric constant, care must be taken in the dimensioning of the off-set surfaces 28b and 28c, and the dielectric slab 34. Using computations well known to the art, the spacing of the surface 280, to which the higher dielectric constant slab 34 is secured, from the lower ground plate 10 is greater than the spacing between the surface of sector 28b and ground 'plate 10 by the amount required to produce equality between the characteristic impedances of a section of trans- I'lllSSlOH line formed by the inner conductor, the surface 28b and ground plate 10 and a section of transmission line formed by the inner conductor, the surface 23c and the ground plate 10. This equality of characteristic impedances insures that the impedance of the device between the terminations 14a and 14b of the center conductor is constant and independent of the rotational position of plate 28. Thecharacteristic impedance of the line section is, in turn, matched to the impedances'of the terminating sections 14a and 14b of the line.

The conductor 14 being located in a semi-circular area below circular plate 28, the portion of the conductor covered by the sectors 28b and 230 can be readily varied by rotating plate 23 about its axis. Such rotation causes a change in phase shift from a maximum amount, when dielectric slab 34 covers the semi-circular area 16, to a minimum amount when sector 28b of plate 28 confronts the area containing conductor 14. The phase shift may be smoothly adjusted between ,these maximum and minimum values with practically no disturbances of the impedances or losses in the device.

Referring to the plan and cross-sectional views of FIGS. 3 and 4, the transmission above-described line elements are shown assembled in a compact unitary package of relatively small size. Primary support for the assembly is a channel member 40 formed of conductive material, the bottom of which serves as the lower ground plate 10 with the upstanding edges serving to enclose two opposite edges of the sandwich assembly of dielectric slab 12,, conductor 14-, dielectric slab 20 and upper ground plate 1%. Dielectric slab 12 is positioned in the channel with the axis through conductor sections 14a and 1412 directed toward the open ends. The sandwich assembly is secured together and to channel member 40 by bolts 42 extending through the corners of the assembly, away from the conductor 14. The open ends of the channel member are closed by a pair of conductive end plates 44 and 46 secured to plate 18 and the bottom of the channel member by four L-shaped members 43.

The present phase shifter being contemplated for use in coaxial transmission line systems, coaxial connectors t and 52 are secured to the end plates 44 and 46, respectively, with their inner conductors 50a and 52a respectively joined to the terminal portions 14a. and 14b of the flat conductor 14. Connection from the inner conductor of the connector to the flat conductor is conveniently made by metallic tabs 54 (FIG. 3) having a width equal .to that of the flat conductor and gradually tapered to the size of the inner conductor of the connector to provide a gradual transition thereby to minimize voltage standing waves at these junctions. FIG. 3 clearly illustrates the positioning of conductor 14 within the semicircular area bounded on one side by the diameter of the circular opening 22 in plate 18.

Referring again to FIG. 4, in which the circular grounding plate 28 is shown in section along a diameter perpendicular to the diameter dividing the under surface into offset planes, the outer rim 28a of the circular plate rests upon the machined shoulder 24 for rotation thereon. To minimize discontinuity in the upper grounding plate the diameter of plate 28 is only slightly less than the outer diameter of shoulder 24 to insure good electrical contact between plate 28 and plate 18, this clearance being exaggerated in FIG. 4. As was noted earlier, when the rotatable plate 28 is in position in the opening in plate 18 sufficient clearance exists between the under surface of dielectric slab 34 and the upper surface of conductor 14 to allow rotation of plate 28 without interference between the dielectric and the conductor. This clearance is exaggerated in FIG. 4 for clarity, a spacing of about 0.002 inch having been found satisfactory in a phase shifter which has been successfully operated.

As shown in FIGS. 1 and 4, the thickness t of lower dielectric slab 12 is substantially greater than the thickness z of the upper dielectric slab 20 and dielectric plate 34 whereby strip conductor 14 lies closer to upper grounding plate 18. With, conductor 14 located nearer the upper grounding plate, signals propagated in the line are influenced to a greater extent by variations of the upper dielectrics, than if the conductor were centrally located. The net result of this unsymmetrical location of the conductor is to maximize the range of adjustment of phase shift of the device, or stated another way, reduces when the propagation velocity of the line is varied from a maximum velocity v to a minimum velocity v Recalling that the propagation velocity v of a transmission line is related to the inductance, L, and capacitance C, of the line, and to the dielectric constant e of the medium between the center conductor and its ground return according to the following equation,

It will be seen from Eq. 4 that the parameters which may influence the range of phase shift, A, of the present device are: the angular frequency of the signal; I, the length of conductor affected by dielectric changes (the reason for using a center conductorofmaximum length);

the square root of the inductance-capacitance product; and

v-v the difierence between the square root of the apparent dielectric constant of the internal dielectrics at the two limiting propagation velocities, v and v 7 Obviously,

the parameters of frequency f, length l, and the inductance-capacitance product, LC, are independent of the location of the center conductor.

In the above-described embodiment, maximum phase shiftoccurs when the dielectric plate 34 lies over the area occupied by the conductor. Thus, at this setting the signal conductor is immersed'in a uniform dielectric medium regardless of its spacing from the ground plates, Consequently, the apparent dielectric constant 6 of the line, at this position of the plate 28, is equal to the dielectric constant of the material of slab 12 and plate 34-,

At minithe position where surface 28b confronts the conductor '14, the dielectric between the conductor and the upper ground plane is air; i.e., the conductor is no longer surrounded by a uniform dielectric. The apparent dielec tric constant at this condition may be designated 6 When the conductor 14 is located relatively close to upper ground plane 28, energy will tend to propagate Z primarily in the region between conductor 14 and ground plane 28. The effective dielectric constant 6 is then essentially that of the air gap between the conductor and the upper ground plane thereby maximizing the factor (V t-V 2) and accordingly the phase shift range 13. Because of the greater spacing between the conductor and the lower ground plane 10, the dielectric in the space between them has little effect on the factor (Va-W2) and therefore can be made of uniform material and held fixed without affecting the phase shift characteristics of the device. Thus, it is seen that the range of phase shift can be maximized by locating the center conductor closer to the upper ground plane thereby allowing the lower ground plane to be fixed, with the attendant advantage of simplifying the physical design.

From the foregoing it is seen that applicants have pro.- vided an improved microwave dielectric phase shifter fabricated of simple and inexpensive components. Since there are no moving electrical contacts to produce impedance discontinuities, the losses are low and the operation reliable. The inner conductor being bonded to and supported by the stationary lower dielectric slab, and the rotatable ground plate being firmly supported, the device cannot be jarred out of alignment when subjected to shock and vibration.

While there has been described what is now considered a preferred embodiment of the invention, numerous modifications may be made without departing from the true spirit thereof. For example, instead of using air and the slab of dielectric 34 as the two dielectrics associated with rotatable plate 28, two dielectric slabs formed of materials having differing dielectric constants may be employed. In this case, also, in order to maintain a constant characteristic impedance independent of the rotational position of plate 28, the under surface of plate 28 would be offset in the manner described earlier, with the dielectric member having the highest dielectric constant secured to the surface furthest removed from conductor 14. Further, while a maximum range of phase shift is obtainable by dividing the two dielectrics on plate 28 along a diameter, the invention is not limited to this configuration, it being possible to divide the dielectrics into other than 180 sectors. For example, for a more limited range of phase shifts the dielectric member 34 might be a 60 pie-shaped sector with the remaining 300 sector having associated therewith the air gap, or vice versa. The device has been described as operable by control knob 32, but it will be appreciated that the device is readily adaptable to servo control. Instead of the control knob 32, a gear may be secured to shaft 30 for coupling to a positional servo for adjusting the rotational position of plate 32 to maintain a prescribed phase shift. Accordingly, the invention is not limited to what has been shown and described except as such limitations appear in the appended claims.

What is claimed is:

1. An adjustable microwave phase shifter comprising a first conductive plate, a first slab of dielectric material supported on said first plate, a conductor supported on the surface of said first slab of dielectric material remote from said first plate, said conductor being of recurvate contour and lying within a semi-circular area, a second conductive plate located on the other side of said conductor and spaced therefrom by a second slab of dielectric material, said second plate and said second slab of dielectric material having a circular opening therein, the diameter of said opening being in register with the diameter of said semi-circular area, a circular conductive plate supported for rotation in said opening, the side of said circular plate confronting said conductor being divided into at least first and second sectors having flat surfaces each parallel to said conductor but differently spaced therefrom, first and second dielectrics having different dielectric constants respectively disposed between said first and second flat surfaces and said conductor, and means for rotating said circular plate relative to said second plate for varying the effective dielectric constant of the dielectrics between said circular plate and said conductor.

2. An adjustable microwave phase shifter comprising a first conductive plate, a first slab of dielectric material supported on said first plate, a conductor supported on the surface of said first slab of dielectric material remote from said first plate, said conductor lying within a semicircular area and having a length greater than the diameter of said area, a second conductive plate located on the other side of said conductor and spaced therefrom by a second slab of dielectric material of a thickness less than the thickness of said first slab of dielectric material, said second plate and said second slab of dielectric material having a circular opening therein, the diameter of said opening being in register with the diameter of said semicircular area, a circular conductive plate supported for rotation in said opening, the side of said circular plate confronting said conductor being divided along a diameter into first and second sectors having flat surfaces each parallel to said conductor but differently spaced therefrom, first and second dielectrics having different dielectric constants respectively disposed between said first and second flat surfaces and said conductor, the spacing between the surface of the sector having the dielectric of higher dielectric constant and said conductor being greater than the spacing between the surface of the other sector and said conductor by the amount required to produce equality between the characteristic impedance of the sections of the transmission line formed by said conductor, said first conductive plate, and the conductive surface of each sector, and means for rotating said circular plate relative to said conductor for varying the effective dielectric constant of the dielectrics between said circular plate and said conductor.

3. An adjustable microwave phase shifter comprising a flat conductor of arcuate contour disposed within a semicircular area, a first conductive plate disposed on one side of said conductor and spaced therefrom by a first slab of dielectric material, a second conductive plate located on the other side of said conductor and spaced therefrom by a second thinner slab of dielectric material, said second plate and said second slab of dielectric material having a circular opening therein a diameter of which is in register with the diameter of said semi-circular area whereby said conductor lies within the area of said opening, a circular conductive plate supported for rotation in said opening, the side of said circular plate confronting said conductor being divided along a diameter into first and second sectors having fiat surfaces each parallel to said conductor and differently spaced therefrom, a slab of solid dielectric material secured to the sector furthest spaced from said conductor, and means for rotating said circular plate relative to said area for varying the effective dielectric constant of the dielectric between said circular plate and said conductor.

4. An adjustable microwave phase shifter comprising a fiat conductor of arcuate contour and disposed within a semi-circular area, a first conductive plate disposed on one side of said conductor and spaced therefrom by a first slab of dielectric material, a second conductive plate located on the other side of said conductor and spaced therefrom by a second thinner slab of dielectric material, said second plate and said second slab of dielectric material having a circular opening therein a diameter of which is in register with the diameter of said semi-circular area whereby said conductor lies within the area of said opening, a circular conductive plate supported for rotation in said opening, the side of said circular plate confronting said conductor being divided along a diameter into first and second sectors having flat surfaces each parallel to said conductor and differently spaced therefrom, a slab of solid dielectric material secured to that sector furthest spaced from said conductor, the spacing of the surface on which said slab of dielectric is supported from said conductor being greater than the spacing between the surface of the other sector and said conductor by the amount required to produce equality between the characteristic impedances of the sections of transmission line formed by said conductor, said first conductive plate, and the conductive surface of each sector, and means for rotating said circular plate relative to said area for varying the eflective dielectric constant of the dielectric between said circular plate and said conductor.

5. An adjustable microwave phase shifter comprising a first flat slab of dielectric material having a first flat conductive plate fixedly supported on one side thereof, a flat conductor of recurvate contour fixedly supported on the other side of said first slab and lying within a semi-circular area, a second flat conductive plate confronting said conductor and spaced therefrom by an amount less than the thickness of said first slab, the side of said second plate which confronts said conductor being divided into first and second sectors each having flat surfaces parallel to said conductor but differently spaced therefrom, a second flat slab of dielectric material secured to that sector furthest spaced from said conductor, the spacing of the surface on which said second slab of dielectric is supported from said conductor being greater than the spacing between the surface of the other sector and said conductor by the amount required to produce equality between the characteristic impedances of the sections of transmission line formed by said conductor, said first conductive plate, and the conductive surface of each sector, and means for rotating said second plate relative to said conductor and said first plate for varying the effective dielectric constant of the region between said conductor and said second plate.

References Cited in the file of this patent UNITED STATES PATENTS 2,755,447 Englemann July 17, 1956 2,915,717 La Rosa Dec. 1, 1959 2,961,622 Sommers Nov. 22, 1960 3,005,168 Fye Oct. 17, 1961 

5. AN ADJUSTABLE MICROWAVE PHASE SHIFTER COMPRISING A FIRST FLAT SLAB OF DIELECTRIC MATERIAL HAVING A FIRST FLAT CONDUCTIVE FIXEDLY SUPPORTED ON ONE SIDE THEREOF, A FLAT CONDUCTOR OF RECURVATE CONTOUR FIXEDLY SUPPORTED ON THE OTHER SIDE OF SAID FIRST SLAB AND LYING WITHIN A SEMI-CIRCULAR AREA, A SECOND FLAT CONDUCTIVE PLATE CONFRONTING SAID CONDUCTOR AND SPACED THEREFROM BY AN AMOUNT LESS THAN THE THICKNESS OF SAID FIRST SLAB, THE SIDE OF SAID SECOND PLATE WHICH CONFRONTS SAID CONDUCTOR BEING DIVIDED INTO FIRST AND SECOND SECTORS EACH HAVING FLAT SURFACES PARALLEL TO SAID CONDUCTOR BUT DIFFERENTLY SPACED THEREFROM, A SECOND FLAT SLAB OF DIELECTRIC MATERIAL SECURED TO THAT SECTOR FURTHEST SPACED FROM SAID CONDUCTOR, THE SPACING OF THE SURFACE ON WHICH SAID SECOND SLAB OF DIELECTRIC IS SUPPORTED FROM SAID CONDUCTOR BEING GREATER THAN THE SPACING BETWEEN THE SURFACE OF THE OTHER SECTOR AND SAID CONDUCTOR BY THE AMOUNT REQUIRED TO PRODUCE EQUALITY BETWEEN THE CHARACTERISTIC IMPEDANCES OF THE SECTIONS OF TRANSMISSION LINE FORMED BY SAID CONDUCTOR, SAID FIRST CONDUCTIVE PLATE, AND THE CONDUCTIVE SURFACE OF EACH SECTOR, AND MEANS FOR ROTATING SAID SECOND PLATE RELATIVE TO SAID CONDUCTOR AND SAID FIRST PLATE FOR VARYING THE EFFECTIVE DIELECTRIC CONSTANT OF THE REGION BETWEEN SAID CONDUCTOR AND SAID SECOND PLATE. 